1. Field of the Invention
The present invention relates to a radio apparatus which is slidably opened and closed in the short-side direction of a housing, and more particularly to a technique to improve the communication characteristics of the radio apparatus.
2. Description of the Related Art
Through the development and improvement of portable communication networks in recent years, large capacity and high-speed data communication has been made possible. As a result, in addition to the telephone-based methods, the PC (Personal Computer) based methods have been increased as the methods of using radio apparatuses, such as a portable radio apparatus (terminal). Along with the diversification of utilization forms of the portable radio apparatus, the shape of the portable radio apparatus itself has also been diversified. As a result, along with the conventional foldable type and the slide type (expressed as long-side direction slide type in this specification for convenience of explanation), a radio apparatus (expressed as short-side direction slide type in this specification for convenience of explanation) which has an easily usable large and wide screen (lateral display), and which has first and second housings configured to be slidably opened and closed in the short-side direction of the housing, has been attracting attention.
In general, when a high frequency signal is supplied to an antenna attached to the small housing, induced current is generated in the vicinity of the antenna of the second housing at which the antenna is arranged. In the case where the first housing is connected to the second housing in a state equivalent to a short-circuit state, induced current is also generated in the first housing similarly to the second housing, and these induced currents contribute to the radiation characteristics of the antenna.
At present, the frequency band mainly used in the radio apparatus is, in many cases, a frequency band corresponding to a wavelength, about one fourth of which (λ/4) is the short-side direction length of the housing, and the above described induced current is generated so as to flow in the direction of λ/4 which is the resonance length of the induced current, that is, in the short-side direction of the housing. It is possible to obtain excellent antenna characteristics by controlling the induced current generated on the housing.
For example, a technique relating to the improvement of antenna characteristics of a long-side direction slide type portable radio apparatus is disclosed in Patent Document 1 as will be described below. According to Patent Document 1, it is possible to widen the frequency band of the antenna and to control the radiation pattern of the antenna. However, since the load connected to widen the band is inductive, and since the load connected to control the radiation pattern is capacitive, the reactance constants of the connected loads need to be different from each other. For this reason, both the purpose of widening the band of the antenna and the purpose of controlling the radiation pattern are difficult to be realized in the same frequency band.
Further, Patent Document 2 as will be described below discloses a technique for improving antenna characteristics of a small and thin long-side direction slide type apparatus in a free space. However, Patent Document 2 originally proposes a new technique for solving the problem that the antenna characteristics cannot be secured.    Patent Document 1: JP Patent Publication (Kokai) No. 2006-203806    Patent Document 2: JP Patent Publication (Kokai) No. 2006-325098